1. Field of the Invention
The present invention relates to a digital recorder, and more particularly, to a transport bit stream recording/reproducing apparatus and method. Although the present invention is applicable to a variety of different devices, is especially applicable to a digital video cassette recorder (DVCR).
2. Discussion of the Related Art
Generally speaking, when information to be transferred through a communication channel is a bit stream of a variable rate, each transport unit of the information is transferred along with a start bit and several stop bits, as shown in FIG. 1. The number of stop bits is controlled to allow the channel to have a desired transport rate.
There has been suggested a method for controlling the number of stop bits in which, as shown in FIG. 2, a controller C controls a variable stop bit generator B, shift register A and storage D according to a control signal applied from an information processing apparatus. Thus, an appropriate control signal is transported to a serial communication channel. Here, the control signal transferred to the serial communication channel varies with the kind of serial communication channel.
Storage D temporarily stores information input from the information processing apparatus prior to its transmission through the serial communication channel. In other words, the information input from the information processing apparatus is stored in the storage, and then it is transported to shift register A according to the control signal received from or transmitted to controller C.
Variable stop bit generator B generates a stop bit according to the control signal received from or transmitted to controller C, as shown in FIG. 1. Shift register A stores and sequentially outputs the data bit from storage D and the stop bit generated from variable stop bit generator B according to the control signal received from or transmitted to controller C.
The above operation converts data to a transport data format and transfers it to a serial communication channel. Information is first recorded in storage D from an information processing apparatus. Here, the clock frequency used may be different from that used in transportation via the communication channel.
The information written in storage D from the information processing apparatus is transmitted to the serial communication channel via shift register A. If there is no data bit to be transmitted, the stop bit generated from variable stop bit generator B is transferred to the serial communication channel via shift register A. Data is always transferred through the serial communication channel.
The above method varies the actual transport rate through the communication channel within a range smaller than the information transport rate while maintaining uniformity of the transport rate. The above method is one way (embodiment) of transferring a variable rate bit stream through the communication channel.
In another method (embodiment) of transferring the variable rate bit stream, there has been suggested a method for multiplexing several variable rate bit streams for digital broadcasting through a variety of broadcasting media such as aerial wave broadcasting, Grand Alliance (which is the U.S. standard in HDTV), cable broadcasting, and satellite broadcasting. Here, the multiplexed bit streams have a constant rate bit stream, which becomes equal to the information transport rate of the broadcasting channel.
When the bit stream transferred at a variable rate is to be recorded or reproduced via a digital recording/reproducing medium, it must be converted into a constant rate bit stream because the medium is capable only of recording the constant rate bit stream. One of the methods for converting the variable rate bit stream into the constant rate bit stream was presented in U.S. Pat. No. 4,914,618. According to this method, the recording rate for the digital medium is fixed at an instant maximum transport rate, and the stop bits are added to the variable rate bit stream. However, this is inefficient because it sharply reduces the net recording capacity of the digital recording medium.
In order to convert the variable rate into a constant rate while maximizing the net recording capacity of the digital medium, a buffer such as an FIFO may be applied. According to this method, the variable rate bit stream is written in the FIFO, and read out at a constant rate. This method may cause errors due to information needed for clock synchronization used in decoding the bit stream, using information contained in the bit stream, making the decoding process unstable in a decoder using the information.
In order to stabilize the method using the FIFO buffer, a time stamp may be added, which is employed in a communication network such as in an asynchronous transfer mode (ATM). A method for adding time stamp in the communication network was suggested in U.S. Pat. No. 5,383,182. Here, a time stamp generator allocates a time stamp to the respective units of information transportation. Adding time stamp to the digital recording medium greatly increases the additional recording capacity required in recording the time stamp. More specifically, a transport packet (the unit of transportation) has a size of 188 bytes, and the time stamp allocated to the transport packet is about 4-6 bytes so that the overhead from the time stamp is 2.1-3.1%.
In short, the above-discussed methods require the variable rate bit stream to be converted into a constant rate bit stream when the variable rate bit stream is recorded or reproduced through a digital recording/reproducing medium. For this, first, there is a method for fixing the recording rate to the digital medium at an instant maximum transport rate and adding the stop bits to the variable rate bit stream. This method is unrealistic, however, because it sharply reduces the net recording capacity of the digital recording medium and makes it difficult to design the hardware for high-speed recording/playback.
Second, there is a method for writing the variable rate bit stream in an FIFO which draws out the oldest data from a data table, and reads it out at a constant rate bit stream. This method maximizes the net recording capacity of the digital recording medium, but causes errors due to information needed for clock synchronization used in decoding the bit stream, i.e., in restoring it for display, using information contained in the bit stream. For this reason, the decoding may become unstable in a decoder using such information, and it may be difficult to obtain compatibility with other sets.
Third, there is a method for adding a time stamp to a transport packet unit. This method yields a stabler operation than the second method, but increases its capacity because the transport packet has a size of 188 bytes and the size of the time stamp is 4-6 bytes, causing the overhead of the time stamp to be 2.1-3.1%.
In the digital recording/reproducing medium, the variable rate bit stream must be converted to a constant rate bit stream. Thus, a technique is needed to eliminate or minimize malfunctions in reproduction which may be caused by rate conversion and recording. However, in recording or reproducing using the above rate conversion of bit stream, the aforementioned problems arise. Therefore, development of a novel method is required for rate conversion of bit stream.